One possible explanation for this might be connected with
the list of repositories used when following dependencies. For example, using the notation of
PPM::Repositories,
assuming ActivePerl 8xx, an ordered repository list of

will cause an error about "no suitable installation
target found" when trying to install a package common
to both the
theory
and
theory58
repositories; for example,

ppm> install Crypt-RSA

This is because the theory repository is for
ActivePerl 6xx packages, and theory58 is for
ActivePerl 8xx packages; however, the ppm utility will
stop at the first matching package found, and then abort if it's
not for the correct platform, even though a correct
platform is available further down in the repository list.

One thing perhaps worth trying is to go through the
repository list and remove any repositories that are
specific to perl-5.6 (if you're using perl-5.8).